1. Field of the Invention
The present invention relates to a hydraulic pressure control device for an automatic transmission, and more specifically, to a hydraulic pressure control device for an automatic transmission having a function to directly supply a line pressure and hold (lock) an engaging pressure by combinations of electromagnetic valves and changeover valves.
2. Description of the Related Art
In recent years, light-weight, compact, and high-torque automatic transmissions have been demanded. This demand has been met by raising a maximum line pressure so as to ensure a torque capacity that does not cause slip of a friction engaging element. However, if the maximum line pressure is raised, the load of an oil pump becomes large. Therefore, there have been efforts to lower the load by providing a throttle valve interlocking with the depressed amount of an accelerator pedal to change the line pressure depending on each torque situation.
Also, if the line pressure is raised, a difference between the maximum torque and the torque of a gear shift region becomes large. Therefore, as descried in JP-A No. 2001-12588, the control gain becomes large and the controllability deteriorates. Thus, these problems are solved in JP-A No. 2001-12588 by performing gear shift in a control range of an electromagnetic valve, etc., and supplying a line pressure to a friction engaging element by using a changeover valve, after completion of the gear shift.
Meanwhile, in JP-A No. 2001-12588, if the changeover valve has stuck (sticking of a valve body) and remains in the state of supplying a line pressure, the friction engaging element cannot be disengaged although a pressure-reducing control valve (or control valve) and a solenoid valve operate normally. In this case, since it is also considered that any interlock is caused unexpectedly, it is necessary to incorporate a fail-safe mechanism that detects sticking of the changeover valve by a hydraulic switch, etc. and forcefully disengages the friction engaging element so that a vehicle can travel.
A hydraulic pressure control device of JP-A No. 2002-266995 is also of a type that performs gear shift in a control range of an electromagnetic valve, etc., and supplies a line pressure to an engaging element by using a changeover valve, after completion of the gear shift. However, the control device is configured such that switching of the changeover valve is performed by an electromagnetic valve (linear solenoid valve), and the electromagnetic valve and the control valve are normally high (normally open), that is, the output pressure thereof is high when power is turned off (refer to FIG. 12).
FIG. 4 is a view showing a state in a case where the changeover valve in a hydraulic circuit of JP-A No. 200-266995 has stuck on the lock side. As indicated by dotted lines in the figure, even when the electromagnetic valve is turned on or off, a line pressure is led to the friction engaging element C-1. Also, FIG. 5 is a view showing a state in a case where the control valve in the hydraulic circuit of JP-A No. 2002-266995 has stuck on the ON-side. Even in this case, as indicated by dotted lines in the figure, even when the electromagnetic valve is turned on or off, a line pressure is led to the friction engaging element C-1. Further, although not shown, even when the electromagnetic valve in the hydraulic circuit of JP-A No. 2002-266995 fails in an ON state, similar to the case in which the changeover valve has stuck on the lock side, a line pressure is led to the friction engaging element C-1.
FIG. 6 is a view showing a state in a case where the electromagnetic valve in the conventional hydraulic circuit of JP-A No. 2002-266995 has failed in an OFF state. In this case, it is natural that the output remains low.
The output should be low in an ON state of the electromagnetic valve, while the output should be high in an OFF state of the electromagnetic valve. However, at the time of abnormality, the output may become high in an ON state of the electromagnetic valve, while the output may become low in an OFF state of the electromagnetic valve. These problems are not caused by installation of the changeover valve. Solutions to these problems have been sought by a hardware failure or a control software failure on a hydraulic circuit using an interlocking valve or a fail-safe valve.